Marraige
Sune and Sharess (and therefore, in almost all cases, their churches, too) wouldn't care about the married or unmarried state of persons indulging in lustful practises. Wherefore performing such short-term marriages wouldn't be clerical policy for them (except perhaps to indulge wanton faithful desiring the thrill of turning a marriage ceremony into debauchery). However, the clergy of Siamorphe, representing a deity catering to the nobility, are concerned both with being popular/useful to nobles, AND safeguarding the rights, privileges, and powers of nobles. Bastard children, loss of virginity, and so-called "immoral" behaviour are matters far more important to noble families than to the general populace. So legitimate but very-short-term marriages allow nobles to ah, enjoy other nobles (or commoners) whilst at the same time protecting the rights, status, and wealth of the participants and their families. For example, a commoner female (or her family) can't easily later demand monies or other compensation from a noble male, in return for her lost maidenhead, when she willingly (the clergy will attest to this, for they test for it, with witnesses and spells) participated in a known-to-be-dusk-until-dawn marriage. In like manner, a noble female can experience the joys of lusty male partners without said partners gaining any claim to becoming part of her family (so she can, er, taste dungsweepers instead of restricting herself to any chinless foolheaded fops who happen to be noble). And so on. A father can promise his daughter in marriage to anyone who can slay the Dread Beast Barthos or recover the Lost Sacred Silver Apple without blindly binding his family to an evil, grasping lout or sacrificing her future and lifelong happiness -- because she can be shrouded in disease-averting, ironguard, and healing spells for a night and just put up with things for the one night. Elder nobles who don't want to share their fortunes or sacrifice their freedom can indulge in brief flings without sacrificing respectability (remember, in most places in the Realms no one deity is paramount with Gond, would be one of the exceptions, so the moral code of no one deity can dominate; whereas in our real world North America Christianity dominates and decides what's "respectable" or not, in the Realms all of the gods have rightful standing, so as ridiculous/insincere/overly convenient as a church-legitimized one-night stand might seem to us, it's not viewed that way by the majority of Faerunians...individuals who cleave to this god or that may sneer or deride or be scandalized, but would have a much harder time than many of our real-world fanatics do in gathering popular support for their personal views). I must preface my answer to your second question with a gentle suggestion (please understand I'm not trying to be critical here) that your question to me, as phrased, betrays a North American Christian-dominated viewpoint. You speak of people "in serious relationships sowing some serious oats" as "lax mores." Though I agree that in general the Realms does follow rather romanticized courtly love-plus-feminine-equality values (or at least, that's what I designed it to have), neither I nor the nigh-immortal everyone, notice, just the Chosen and other live-for-many-centuries folks, and nobles and royalty who consider themselves 'above' laws and social rules, or to be the people who set such laws and rules fictional Realms characters I've created view "sleeping with" people as being incompatible with having deep, committed relationships with someone else. So they don't see it as "lax" at all (and by the way, neither did a LOT of real-world American people of a certain generation, during the 1960s/Woodstock generation -- making love to Person A was seen as having nothing at all to do with being life-bonded to Person B). Yes, The Simbul and Elminster DO love each other. Deeply. Yet neither of them would define faithfulness to the other as having anything at all to do with sex. So, yes, "swinging" between committed or married couples isn't seen as Bad by a lot of Faerunians, in many places and situations (though among most citizens across the Realms, it would be). fashionable among some noble classes and a LOT of "wannabe noble" rich, rising merchants, and frowned upon in places with small, stable populations where warfare or monster predations haven't forced folk into desperate survival measures (telescoping survivors down into a single extended family of multiple husbands and wives, for example). I make no apology for this mental separation between love and lust. Outliving lover after lover, family after family, (many of) your own children, realm after realm, and so on will do that to you. You grab physical love when you can, and search for long-term partners with a desperate hunger. Or at least, that's how I've chosen to define the effects of lonely longevity on persons trying (and usually failing) to remain sane. If I was publishing the "uncensored" Realms, in fiction, most of my liches would be desperate to have physical relations with adventurers, not kill them. Think about it. I'm well aware that many gamers, reviewers, academics, and persons with only a casual understanding of fantasy roleplaying games have labeled me as some sort of pervert or (at best) immoral "dirty old man" for holding such views (strange, that 'old' bit, considering I was examining these issues and settling on this particular viewpoint when I was about fourteen), and of course the shortage of centuries-old real-world people to examine makes the point moot, but I'd like more folks to consider that situation and come up with alternative desires and drives that might dominate such long-lived characters (pure power is one, striving for immortality at the cost of humanity through undeath is another the game rules present to us). Interesting, yes? The Realms has many such 'deep waters' places, awaiting those who question deeply enough.